The present invention relates generally to systems and methods for inspecting a structure. More particularly, the embodiments disclosed herein relate to systems and methods for non-destructive inspection of parts made of fiber-reinforced composite material.
Fiber-reinforced composite materials comprise fibers embedded in a matrix material, such as thermoset and thermoplastic polymer resins. As is well known, a composite laminate is constructed with multiple (e.g., 20) plies. For a conventional ply of a flat laminate, each ply has fiber paths which are described entirely by ply angles selected from the group consisting of 0°, ±45° and 90°. However, pairs of 0° plies or ±45° plies in a conventional laminate may be replaced with pairs of steered-fiber plies, the plies of each pair being balanced. The steered-fiber plies of each pair may be separated by one or more intervening conventional plies.
It is increasingly common practice to manufacture aerospace parts from lightweight, high-strength fiber-reinforced composite materials, such as carbon-fiber epoxy composites. Each carbon fiber strand is a bundle of many thousand carbon filaments. A single such filament is a thin tube with a diameter of 5-8 micrometers and consists almost exclusively of carbon. Thick, complex carbon-fiber epoxy composites are difficult to uniformly produce, and susceptible to internal movements and shifts during curing. These problems mitigate the strength and reliability of fiber-reinforced composite parts, and become more pronounced during curing of thick parts such as helicopter blades or aircraft structural joints.
Evaluation of such finished composite parts is difficult. The qualification of complex parts after manufacturing requires non-destructive evaluation (NDE) that is able to identify the occurrence and frequency of such defects as air bubbles/voids, ripples or waves in the fiber alignment, and balling or bunching of fibers, especially near the ends of layout fiber runs.
Common NDE methods have difficulty in imaging subtle fiber orientations in thick epoxy parts. Present methods inject energy—acoustic, electromagnetic, x-ray radiation or magnetic field—and then infer the internal structure of complex parts by the resulting energy coming back out. Ultrasound works well for finding bubbles and arguably epoxy pooling, but is insensitive to carbon fiber waviness. X-rays are insensitive to distinguishing carbon fibers and epoxy. Carbon fibers absorb and dissipate electromagnetic energy, precluding high-resolution studies of thick parts. Magnetic resonance imaging is expensive, onerous and requires that the entire part be placed into the imaging machine.
There is a need for systems and methods for accurate non-destructive inspection of fiber-reinforced composite parts for incorporation into larger structures during assembly.